Many of us having had OC knows about the challenges with mechanical hum from the transformer often caused by DC. In a smaller room this can get problematic.
To those who have had these issues and then moved on to 3-series say 332, 250/350. Did you notice the hum levels disappeared or are they still as present as with OC boxes? I believe Naim made a statement on that this issue was reduced or removed with NC?
No - I had fairly noise hum from my NC250. Completely resolved with an iFi DC blocker with no detriment to sound. If I understand correctly, hum is a possible issue with any large toroidal transformer and the NC series still uses these
They do hum the 350 but very minimal . Dont notice unless you put your ear on them
Used to have occasional low hum noticeable when close to speakers. After installing a dedicated power line for the HiFi, the hum has gone.
I bought an NC 222 which I took back to shop to check the hum as I thought it was faulty.
Staff couldn’t hear it - until them to be quite for 20 mins.
They then heard it it.
We fitted a PBX 300 to it and the hum hum moved from the NSC 222 to the NPX 300.
I now have both but fitted a Audiolab DC blocker.
Whilst there is hum from the units, it’s significantly reduced.
The DC blocker also brought sonic benefits well below what Naim would charge for similar benefits.
I find the New Classic gear to have by far the loudest him of any Naim gear I’ve owned, especially the 250, so sadly I don’t think they have made any progress on this issue.
My understanding is that it is an inherent risk of large toroidal transformers and therefore progress is not really possible. Usual cause is some other device in the house on the mains that ‘pollutes’ the AC supply and causes AC offset which upsets these transformers
My OC 250 had a hum,my NC 250 hasn’t.
It does seem a bit random - I had no hum on my Nova and significant hum on my NC 250
Spot on as my set up suffered moving from 20 year old house to 100 year old plus house. Silence to low hum worse on separate Nait 2 setup despite been serviced and using audiolab DC. Must remove that to see if sound affected.
Fridge needs to stay on though
I’ve had OC boxes that hummed a lot and some that didn’t hum at all so maybe it is still random if you get it or not even with NC then. Which is also quite strange given they should have similar designs. My current 555PSDR is totally silent behind a Puritan 156. But the former 500PS/552PS had problems which I believe @Dan_M is looking into right now. It is very relieving with a totally hum free system to me so not knowing what you get is a bit so so. How come not all boxes behave similar if the design is similar. Do they get worn out with time and travel between customers. Transformers gets loose and vibrate more? 
I have a NSC222 and NAP250 NC housed in a cabinet along with a Rega P8 power supply, Tom Evans phono and power supply and Naim Core. The P8 sits on top of the unit.
When I first installed everything there was no hum however when I added a Naim Powerline to the 250 this did induce so hum. However, this was resolved with cable dressing and all is hum free again.
If I put my head close to the boxes when nothing is playing, I hear a very faint hum, but if I am more than around 30 cm from the rack, I hear nothing.
Do you mean mechanical hum from a transformer or hum through the speaker? This thread is only about mechanical hum from transformers 
My 2 x 555PSDR and 552DR have no hum.
These are much newer units than the 2002 500DR, which was DR’d in 2017. The 500DR hums even on the Puritan 156 block, albeit not as much.
The 500DR has hummed since I bought it almost 3 years ago.
I have a new separate mains now and has made no difference to the hum.
It’s due a service in 6 years or so. Am okay living with the slight hum.
My brand new NSS333 was very noisy and we had to change the transformer. The Nac 332 was noisy as well but the problem was solved with a dc blocker. No noise from Nap 350. Anyway they were all early serial numbers. Don’t know if current models are better.
I remember a guy here who changed his fridge to reduce hum. Maybe it’s @drago .
I find these differences between boxes very strange. Why do some hum and others don’t.
There is a certain randomness to it
Indeed. The NSS 333 was really noisy. Not only at home but also at my dealer shop which is about 50 km away. But after having changed the transformer it’s absolutely silent.